In the case of transmitting a pulse signal, it is desirable in the use of a transmission line as well as associated circuit devices that the transmission pulse signal is D.C. balanced and that timing information is easy to extract from the signal at the receiving end. Further, in the use of optical communication circuits and the like, two-level transmission pulses are desirable from the viewpoint of the nonlinearity of the light source.
As a pulse signal transmission system fulfilling these conditions, it has been known to use a pulse signal in which one of the original binary information levels "0" and "1" is encoded into a pulse "11" or "00" and the other is encoded into a pulse "01" or "10"; in other words, one binary information level is converted into alternate polarities at a period T and the other is provided at a period T/2. Hereinbelow, such a pulse signal shall be referred to as a "two-level AMI (Alternate Mark Inversion) signal". As compared with conventional three-level bipolar codes, the two-level AMI codes have one-half of the transmission pulse period, namely, two times the timing frequency, and have twice the transmission speed (bandwidth) required of a transmitter and receiver. It is therefore unfavorable when the clock rate of the original information signal is high (for example, 100 Mb/s or above) since this requires extremely high speed circuitry.
As a measure for eliminating this problem, the two-level AMI signal is subjected to a duo-binary shaping in a receiving circuit portion of the system. This processing is performed with a low-pass filter whose frequency characteristic is cosine-shaped. Equivalently, the filter has the effect of adding the two-level AMI signal and a signal obtained by delaying this two-level AMI signal by 1/2T. A waveform is obtained after the processing which takes the form of a three-level bipolar signal. Since the duo-binary shaped signal arises at the same clock rate T as that of the original information signal, it has the advantage that the transmission band beciomes one-half of that in the case of transmitting the unmodified two-level AMI signal. However, it has the disadvantage that, when information "0's" succeed one another in the original information, a stable timing signal cannot be effectively extracted at the receiving end.
In the case of the three-level AMI signal, when such information "0's" succeed one another, a method (hereinbelow, called the "zero substitution method") is sometimes relied on wherein the successive "0's" are replaced in the transmission by a waveform pattern which does not conform with a specified AMI law, and on the receiving side, the waveform pattern is utilized as timing information. In addition, the part of the signal replaced by the specified waveform pattern is reconverted into the succession of "0's" provided by the original information. This method, however, cannot be applied to the case of a two-level AMI signal. More specifically, even when the zero succession part of the signal is replaced on the transmitting side by a special pattern which does not conform with the code conversion rules of the two-level AMI signal, undefined pulses arise in the duo-binary shaped three-level AMI signal. It is therefore difficult to properly identify the zero substitution part and to properly convert this part into the zero succession signal provided in the original information.